In a gas turbine engine, an extremely hot (about 2200.degree. to 2500.degree. F.) gas jet impinges upon a turbine, causing the turbine to spin and thereby do work. It is well known to cool the turbine blades by ducting an airflow through them internally. However, this ducting is expensive in the sense that the cooling airflow is diverted from a power-producing airflow. Therefore, it is desirable to reduce or eliminate turbine blade cooling when it is not needed.